1. Field of the Invention
The invention relates to a fuel cell system for controlling the gas supply/discharge by estimating the impurity concentration in a fuel cell.
2. Description of the Related Art
Fuel cell systems applicable to fuel cell automobiles or the like are known. The fuel cell system includes a fuel cell stack as a main component, and the fuel cell stack has an anode and a cathode. Hydrogen is supplied as a fuel gas to the anode, and the air is supplied to the cathode. Further, the fuel cell system includes components such as a tank for storing the fuel gas, e.g., the hydrogen to be supplied to the anode, and a pump for circulating the exhaust gas containing the unconsumed fuel gas back to the anode. In the fuel cell stack, the hydrogen reacts with oxygen in the air to generate electricity.
It is known that as the progress of reaction in the fuel cell, nitrogen in the cathode gas (air) and water produced in the reaction move from the cathode toward the anode through an electrolyte membrane. Therefore, the partial pressure of the nitrogen or water vapor (hereinafter also collectively referred to as the “impurities”) increases, and the concentration of the fuel gas (hydrogen) decreases. As a result, the power generation performance of the fuel cell is lowered undesirably.
In an attempt to address the problem, generally, a discharge valve provided in a discharge passage on the anode side (hereinafter also referred to as a “hydrogen system”) is opened to discharge the gas containing the unconsumed hydrogen and the impurities. For example, Japanese Patent Application Laid Open No. 2003-168467 discloses a technique in a fuel cell system in which an air pump of a fuel cell is operated at a target rotational speed, and an air regulator valve is used for pressure adjustment to a target pressure. In the disclosed technique, the atmospheric pressure is estimated based on the rotational speed and the valve position of the pressure regulator valve for correcting the discharge interval of a hydrogen discharge valve. Further, in a technique disclosed in Japanese Patent Application Laid Open No. 2002-289237, if the impurity concentration in a hydrogen system increases, a hydrogen off gas (consumed gas used during generation of electricity) is discharged from the hydrogen system to decrease the impurity concentration.
However, in the art disclosed in Japanese Patent Application Laid Open No. 2003-168467, the discharge interval is controlled only based on the flow rate of the air without taking the gas state in the hydrogen system into consideration. Further, in the art disclosed in Japanese Patent Application Laid Open No. 2002-289237, the impurity concentration is estimated only based on the elapsed time since the start of operation of the fuel cell system or the hydrogen concentration detected by a hydrogen concentration sensor, and the discharging operation in the hydrogen system is performed in accordance with the estimated impurity concentration. Therefore, in the control techniques disclosed in Japanese Patent Application Laid Open No. 2003-168467 and Japanese Patent Application Laid Open No. 2002-289237, since the current state in the hydrogen system is not taken into consideration in estimating the impurity concentration, the unconsumed hydrogen is discharged wastefully. Therefore, for example, fuel economy of the fuel cell is lowered undesirably.